Gas exhausted from internal combustion engines such as diesel engines or other combustion devices contains a large amount of particulate matter (PM) having soot as main component. Since the PM directly released into the atmosphere causes environmental pollution, a diesel particulate filter (DPF) for trapping the PM is mounted at an exhaust system of exhaust gas.
A honeycomb structure having a porous wall dividedly forming a plurality of cells that will be channels of fluid (exhaust gas and purified gas) has been used as the DPF. Such a honeycomb structure is used as a sealed honeycomb structure (honeycomb filter) by mounting sealing portions at openings of predetermined cells (inlet cell) disposed on an end surface of an outlet side of fluid (purified gas) and at openings of remaining cells (outlet cell) disposed on an end surface of an inlet side of fluid (exhaust gas).
If the exhaust gas flowing into the inlet cell, the particulate matter in the exhaust gas is trapped at a wall when the exhaust gas passes through the wall and the purified gas from which the particulate matter is eliminated flows out from the outlet cell according to the sealed honeycomb structure.
To prevent damage due to excessive thermal impact or mechanical impact occurring at intersecting points of the walls in conventional honeycomb structures, a ceramic honeycomb structure where a cross-sectional shape of some channels (cells) vertical to an axial direction is such that R portions of arc shape are formed at diagonally confronting corner portions is developed (e.g., please refer to patent document 1). In addition, a honeycomb structure porous where fillets are formed at corner portions of cells dividedly formed by porous walls so as to reinforce the cells is also developed (e.g., please refer to patent document 2). According to such honeycomb structures, thickness of an intersecting part of the walls increases, and thereby, mechanical strength (i.e., durability) may be improved.